1. Field of the Invention
The present invention relates to an improved process for making selected doped barium and strontium hexaferrite particles of the formula (I): EQU AFe.sub.12-r-s M.sub.r N.sub.s O.sub.19 (I)
wherein A is barium or strontium; M is cobalt, zinc or nickel; N is titanium or ruthenium and r and s are individually selected from about 0.1 to about 1.2.
2. Brief Description of the Prior Art
Doped barium and strontium hexaferrite particles may be used in a wide variety of magnetic recording media (e.g. audio tape, video tape, computer tape and computer disks). It is believed that these particles are especially suitable for high density recording media (i.e. where high density information storage is required). To be acceptable for these applications, the hexaferrite particles must possess a certain combination of physical and chemical characteristics. The important product parameters currently looked at by workers in this technical area include the following:
(a) average particle size PA1 (b) particle size distribution PA1 (c) particle shape (i.e. aspect ratio) PA1 (d) specific surface area (m.sup.2 /gm)(BET) PA1 (e) specific magnetization (.delta..sub.s) PA1 (f) remanence magnetization (B.sub.r) PA1 (g) particle coercivity (H.sub.c) PA1 (h) type and amount of dopants
While these specific product parameters may be different for individual end uses, each generally must be within an acceptable range for most recording applications.
For example, the average particle diameters are generally desired for most recording applications in the range of from about 0.01 to about 0.3 microns. Particles having an average diameter over about 0.3 microns are difficult to uniformly disperse on a magnetic recording substrate. Particles below about 0.01 micron diameter do not possess the desired ferromagnetic properties. For high density type recording media, it has been found that average particle diameters in the range from about 0.03 to 0.1 microns are especially preferred.
In addition, the particle size distribution should be as narrow as possible to obtain good uniform dispersibility on the recording substrate. If the particles sizes vary over a wide range, the resulting dispersed film of particles on the recording substrate will be uneven. It would be particularly advantageous that at least 95 percent of all of the particles, more preferably, at least 99%, be in a range from about 0.01 to about 0.2 microns.
It is also desired for most recording applications that the doped hexaferrite particles be platelet in shape. This means that the diameter/thickness ratio (i.e. ratio of the average diameter of the c-plane of the crystal of the particles to the thickness of the c-axial direction of the crystals) is in the range from about 2:1 to about 10:1. This diameter/thickness ratio is also known as the aspect ratio. Particles having lower aspect ratios (e.g. 2:1 to 4:1) have higher packing densities in magnetic media (i.e. more particles per unit volume). Particles having higher aspect ratios (e.g. 8:1 to 10:1) may be more easily oriented in the longitudinal plane of the recording media. However, high aspect ratio particles may have problems where smoothness of the surface of the recording media is important.
Generally, particle surface areas are dependent upon the combination of the average particle size, the size distribution and the aspect ratio. Specific surface areas (BET) in the range from about 15 m.sup.2 /gram to 42m.sup.2 /gram are usually desired for high density magnetic recording.
Furthermore, it is generally desirable that the Particles have a specific magnetization (.delta..sub.s) of at least about 45 "emu/g". in a magnetic field of 10K Oe. When the specific magnetization values are below about 45 emu/g, the signal output is significantly reduced in the magnetic recording media. For most high density magnetic recording media applications, specific magnetization (.delta..sub.s) values of at least about 50 emu/g in a magnetic field of 10K Oe are generally suitable.
Acceptable remanence magnetization (B.sub.r) values of the particles are generally in the range from about 18-40 emu/g in a magnetic field of 10K Oe. If the B.sub.r values of the particles are too low, the signal output of the resulting media may not be strong enough for recording purposes.
It is also generally desirable for most recording applications that the coercive force of the hexaferrite particles in the magnetic recording media should be in the range from about 200 to about 1500 Oe. For most high density recording applications, this range is suitable in the range from about 400 to about 1100 Oe. It is known that the partial replacement of iron atoms in the ferrite crystalline structure with selected dopants will lower the coercive force from unacceptably high levels (e.g., about 3000 Oe or higher) to these desired levels. However, the use of these dopants in too large quantities may also lower the specific magnetization levels to below the above-noted acceptable levels.
It has been found that certain pairs of dopants provide acceptable lowering of the coercive force of the hexaferrite particles. Cobalt and titanium are the most widely used dopant pair. When this dopant pair has been used, generally about 0.6-1.0 moles of each are used to replace iron atoms per mole of hexaferrite.
Several processes for making undoped or doped barium or strontium hexaferrite particles are known. One method is the ceramic or "dry" method. In this method, barium or strontium oxide and iron (III) oxide (with or without dopants) are mixed in the desired mole ratio, and the mixture heated to a high temperature (e.g., at least 1000.degree. C.) to form a spinel or hexagonal barium or strontium ferrite. See U.S. Pat. No. 4,425,250. The formed particles have an average particle diameter of several microns. These relatively large particles are then extensively milled (e.g. ball milling or grinding) to obtain an acceptable average particle size. However, this milling process results in an unacceptable broad particle size distribution, which may cause a non-uniform dispersibility on the magnetic recording media and high noise levels in the end use recording products. Furthermore, it has also been reported that the milling produces crystal defects in the particles, creating poor magnetization properties and high noise characteristics. Still further, it is not easy to remove impurities already present or formed during this dry process.
Another method is the glass melt or crystallization process wherein iron particles and barium or strontium carbonate are mixed (with or without dopants) and melted in a glass forming chemical, for example, one containing sodium tetraborate. The glass is then cooled and calcined between 700.degree. C. and 1100.degree. C. See U.S. Pat. Nos. 4,279,763; 4,341,648; and 4,543,198. The glassy substance prevents the ferrite particles from sintering during the calcination process. The glassy substance is then removed by acid washing (e.g. acetic acid) and the hexaferrite product is then recovered. This process is relatively expensive to run because of the need for large amounts of glass-forming chemicals, the high temperatures used for making the glass melt and the need for special equipment and materials of construction (e.g. platinum). Furthermore, this process requires the extensive handling and recycling of large amounts of glass-forming compounds. Still further, the reaction of the acid with the glassy substance in the acid washing step may result in a residue in the hexaferrite particles, which may interfere with other constituents in the end products (e.g. polyethylene binders).
Still another method of making these particles is the autoclaving or hydrothermal process. In this process, Fe(OH).sub.3 or other iron compounds, a barium or strontium compound, and with or without dopants such as a cobalt compound and a titanium compound are autoclaved in an aqueous alkaline suspension at 250.degree. C. to about 400.degree. C. to produce precipitated barium hexaferrite particles. See U.S. Pat. Nos. 4,512,906; 4,529,524; 4,539,129; 4,548,801; 4,561,988 and 4,585,568. The particles produced from the autoclave reactor generally have undesirably low specific magnetization (.delta..sub.s) values (e.g. 30-45 emu/g on a magnetic field of 10K Oe). Thus, for high density magnetic recording media applications, these particles require further processing (e.g. calcination and milling). This further processing increases the cost of making the particles and the milling may be harmful to the particle crystals for the reasons stated above.
Co-precipitation processes have also been employed to prepare barium and strontium hexaferrites. Such processes normally comprise first admixing in an aqueous alkaline suspension barium or strontium ions, iron ions, hydroxide ions, and carbonate ions (with or without dopants) to produce an iron oxide or an iron hydroxide and a barium or strontium carbonate co-precipitate, separating the co-precipitate from the reaction mixture, washing, spray drying, and then calcining the co-precipitate at selected high temperatures to form crystalline barium or strontium hexaferrite particles. See U.S. Pat. Nos. 4,120,807 and 4,440,713. However, it has been reported that these co-precipitate processes have several disadvantages. One is that the co-precipitates produced generally have a very small average diameter (i.e., less than 0.05 micron), which makes the particles very difficult to separate from the reaction mixture. This may also result in a lengthy separation time. A further disadvantage arises when filtration is used as the separation technique. Some of the very small barium or strontium carbonate portions of the co-precipitated particle may pass through the filter, thus undesirably altering the mole ratio of the barium or strontium hexaferrite compound. Still another disadvantage may arise during the calcination step. The particles may combine or fuse or sinter together during this heating step to form unacceptably large particles of one or more microns in diameter. These unacceptably large particles require milling and the above-described problems associated with milling may occur here.
One other known process for making barium or strontium hexaferrite particles is the "salt flux" or "salt melt" process. In this process, a mixture of a barium or strontium compound and an iron compound are first mixed together in a salt (e.g. BaCl, NaCl). The mixture is then dried (e.g. by spray drying) and calcined at or above the melt point of the salt to form hexaferrite particles in the salt melt. The salt is then dissolved and separated from the hexaferrite particles. See U.S. Pat. Nos. 3,793,443; 3,810,973 and 4,116,752. However, this process is the "dry" method in that it generally produces particles which are too large (i.e. above 0.5 microns) for use in high density magnetic recording media.
All of the above-noted U.S. Patents are incorporated herein by reference in their entireties.
In spite of all of these prior art processes for making undoped and doped barium and strontium hexaferrite particles, there is still a need for a better and more inexpensive process which is able to produce particles having product parameters above-noted general ranges, yet is flexible enough to obtain desired specific physical and chemical parameters needed for individual end uses without the problems associated with the known prior art processes. The present invention is believed to meet that need.